1. Field of the Invention
The present invention relates to a printing apparatus and a method for printing images, and more particularly, to a printing apparatus and a method for forming high-quality images by forming a plurality of dots on a printing medium.
2. Description of the Related Art
Various types of printing apparatus are widely used in printers, copying machines, fax machines, and other image recording devices. Printing apparatus forms images by forming a plurality of dots on the printing media. The printing media may be paper, plastic film, clothing, etc. The printing apparatuses may use different kinds of method to perform the printing, such as dot-matrix printing, thermal transfer printing, dye-sublimation printing, ink jet printing, and laser printing, . . . etc. Generally, the thermal transfer printing and the ink jet printing methods are widely applied due to their capability of producing high quality images at reasonable cost.
Referring to FIG. 1, a printing apparatus has a print head 11 that comprises a group of printing elements 12. When the print head 11 scans over a printing medium, the printing elements 12 form a plurality of dots on the printing medium to form an image. In this example, when the print head 11 scans in the scanning direction, an image of dot patterns 13 of Arabic numeral “2” is formed by the printing elements 12, as shown in FIG. 1.
Referring to FIG. 2, for most ink jet printing products nowadays, a step motor 22 is used to provide a power so as to drive the print head 21 to move along a scanning direction over the printing medium. As shown in FIG. 2, the step motor 22 drives the print head 21 to move in a predetermined scanning direction by using the mechanism such as belt 23. When the print head 21 scans in the predetermined scanning direction, the printing elements 12 are driven to fire ink droplets during predetermined time interval to form dots on desired positions of the printing medium.
The purpose of the step motor is to drive the print head by a power transmission device so that the print head can scan over the printing medium at a constant speed. The power transmission device may be belt. Step motors are generally used because their cost is lower and they're comparatively easy to control. However, the variation of different components, including step motors, control chips, and belts, may cause a cyclic variation of the print head's moving speed. Therefore, the print positions of dots would cyclically deviate from the desired positions. FIG. 3a shows a cyclic unevenness phenomenon of images due to the cyclic variation of the scanning speed of the print head. The positions of dots deviate cyclically. FIG. 3b shows distance between two adjacent dots varying cyclically due to the cyclic variation. Such deviations cause noticeable unevenness of the printed image, and thus deteriorate the printing quality.
In the U.S. Pat. No. 6,142,598 issued to Iwasaki et al., a method is provided to decrease the cyclic unevenness of printed images by cyclically changing the time intervals between adjacent ink ejections. A predetermined cyclic deviation amount is added to the driving time interval between dots. Thus, a cyclic shift amount appears in the print position of dot. Referring to FIG. 4a, it shows shift amounts of dots when a print head moves at a constant speed across the medium. For a printing apparatus with the defect of cyclic unevenness, the Iwasaki's method shifts the deviated positions of dots. Referring to FIG. 4b, it shows the effect on cyclic deviation after Iwasaki's method is employed. FIG. 4c further shows the effect of the method on the distance between two adjacent dots. As shown is FIG. 4C, Iwasaki's method may decrease the original unevenness, however, the improvement is not satisfactory and further improvement is still desired.